Expression and neural control of myogenic regulatory factor genes during regeneration of mouse soleus.

Given the importance of the myogenic regulatory factors (MRFs) for myoblast differentiation during development, the aims of this work were to clarify the spatial and temporal expression pattern of the four MRF mRNAs during soleus regeneration in mouse after cardiotoxin injury, using in situ hybridization, and to investigate the influence of innervation on the expression of each MRF during a complete degeneration/regeneration process. For this, we performed cardiotoxin injury-induced regeneration experiments on denervated soleus muscle. Myf-5, MyoD, and MRF4 mRNAs were detected in satellite cell-derived myoblasts in the first stages of muscle regeneration analyzed (2--3 days P-I). The Myf-5 transcript level dramatically decreased in young multinucleated myotubes, whereas MyoD and MRF4 transcripts were expressed persistently throughout the regeneration process. Myogenin mRNA was transiently expressed in forming myotubes. These results are discussed with regard to the potential relationships between MyoD and MRF4 in the satellite cell differentiation pathway. Muscle denervation precociously (at 8 days P-I) upregulated both the Myf-5 and the MRF4 mRNA levels, whereas the increase of both MyoD and myogenin mRNA levels was observed later, in the late stages of regeneration (30 days P-I). This significant accumulation of each differentially upregulated MRF during soleus regeneration after denervation suggests that each myogenic factor might have a distinct role in the regulatory control of muscle gene expression. This role is discussed in relation to the expression of the nerve-regulated genes, such as the nAChR subunit gene family. (J Histochem Cytochem 49:887-899, 2001)

Effects of eccentric treadmill running on mouse soleus: degeneration/regeneration studied with Myf-5 and MyoD probes.

AIM: The aim of this report is to show that eccentric exercise under well-controlled conditions is an alternative model, to chemical and mechanical analyses, and analyse the process of degeneration/regeneration in mouse soleus. METHODS: For this, mice were submitted to a single bout of eccentric exercise on a treadmill down a 14 degrees decline for 150 min and the soleus muscle was analysed at different times following exercise by histology and in situ hybridization in comparison with cardiotoxin-injured muscles. RESULTS: We analyse the regenerative process by detection of the accumulation of transcripts coding for the two myogenic regulatory factors, Myf-5 and MyoD, which are good markers of the activated satellite cells. From 24 h post-exercise (P-E), clusters of mononucleated Myf-5/MyoD-positive cells were detected. Their number increased up to 96 h P-E when young MyoD-positive myotubes with central nuclei began to appear. From 96 to 168 h P-E the number of myotubes increased, about 10-fold, the new myotubes representing 58% of the muscle cells (168 h P-E). CONCLUSION: These results show that this protocol of eccentric exercise is able to induce a drastic degeneration/regeneration process in the soleus muscle. This offers the opportunity to perform biochemical and molecular analyses of a process of regeneration without muscle environment defects. The advantages of this model are discussed in the context of fundamental and therapeutical perspectives.